PFT
There are a few things to determine before performing the pulmonary function test: Variables
Height: A tall person has bigger lungs than a short of person.
Age: Lung function declines after the age of 25 years, regardless of whether the person is a smoker or not.
Gender: Males and females will have different-sized lungs. This is independent of the height.
Race: A white man will have his navel down lower than a black man. Black men will have smaller lungs then white men.
In interpreting PFTs first look at FVC to see if it is 80% or more of the predicted value.
If the FVC is greater than 80% then there is no restriction.
The next step is to determine if the FEV1/FVC > 0.7. If the answer is yes then there is no obstruction. If it is less than 0.7 then you have obstruction.
If the FVC is less than 80% then there is either restriction or obstruction with air trapping.
Next, determine the total lung capacity (TLC). If it is more than 80% predicted, then you have obstruction with air trapping. If it is less than 80% predicted, then you have a restriction.
When considering severity of obstruction consider Gold Classification or Stages for COPD: I, II, III, and IV. The parameter to consider here is only FEV1.
Stage I: FEV1 is 80-100.
Stage II: FEV1 is 50-80.
Stage III: FEV1 is 30-50.
Stage IV: FEV1 is 0-30 or if there is respiratory failure with elevated PCO2 level.
Also when considering obstruction you need to assess for reactivity. The ATS criteria for reactivity is FEV1 or FVC change in percentage of postbronchodilator to prebronchodilator greater than 12% and 200 mL. If you do not have either of the two then it is a non-reactive airway.
DLCO: Whether or not if it is >80%.
If the DLCO is more than 80% of predicted then you have normal membrane surface area.
If it is less than 80% of predicted then it is an abnormal membrane surface area.
If so the next thing to look at is his DLCO/Va (alveolar ventilation) to see if it is greater than 80% of predicted.
If yes, it denotes extrinsic lung disease (scoliosis or GBS).
If no, it denotes intrinsic lung disease (pulmonary fibrosis or COPD).
In the presence of restriction and normal DLCO = extrapulmonary disease (NMD)
In the presence of normal lung mechanics and reduced DLCO = Pulmonary vascular disease.
PFT profiles:
FEV1: normal
FEV1/FVC: normal
VC: low
TLC: low
RV: low
DLCO: normal
Interpretation: restrictive lung disease - extraparenchymal (obesity, kyphosis)
FEV1: low
FEV1/FVC: low
VC: high
TLC: high
RV: high
DLCO: low
Interpretation: obstructive lung disease - COPD, emphysema
FEV1: low
FEV1/FVC: low
VC: high
TLC: high
RV: high
DLCO: normal
Interpretation: obstructive lung disease - asthma
FEV1: normal
FEV1/FVC: high
VC: low
TLC: low
RV: low
DLCO: low
Interpretation: restrictive lung disease - pulmonary fibrosis
DLCO: It provides an estimate of the rate at which O2 moves by diffusion from alveolar gas to combine with hemoglobin in the RBC. It is interpreted as an index of the surface area engaged in alveolar-capillary diffusion. Measurement of diffusing capacity of the lung is done by having the person inspire a low concentration of carbon monoxide. The rate of uptake of the gas by the blood is calculated from the difference between the inspired and expired concentrations. The test can be performed during a single 10 second breath holding or during 1 minute of steady-state breathing. The diffusing capacity is defined as the amount of CO transferred per min/per mm Hg of driving pressure and correlates with oxygen transfer from the alveolus to the capillaries.
▲ DLCO: ▲ blood volume, alveolar H'ge, ▲pulmonary blood flow, airway edema, CHF, polycythemia.
▼ DLCO: Primary parenchymal disorders, anemia, and removal of lung tissue
Preoperative Pulmonary Function Assessment
The objective is to establish that after surgical resection of the lung for a tumor, there will be sufficient pulmonary reserve to keep the patient comfortable, and will not become a respiratory cripple.
You should always evaluate the patient to determine whether he could withstand pneumonectomy even if radiologically only a lobectomy or limited resection is contemplated. On thoracotomy, a surgeon may be forced to do pneumonectomy because of an unexpected node over the pulmonary artery. If you have decided the patient cannot withstand pneumonectomy, this should be addressed with the surgeon ahead of thoracotomy.
Step 1: Routine PFTs. If the patient meets the following criteria, no further workup is necessary:
If these criteria were met and the patient were to have pneumonectomy, he would be left with at least 1 liter of FEV1 in the residual lung.
Step 2: If the patient does not meet the above criteria on routine PFT, and if the FEV1 volume is less than 2 liter, we need to perform split lung function testing. Lungs with tumor may not be contributing to total FEV1 volume and thus removal of it may not significantly affect pulmonary function. On the other hand, in some patients the diseased lung is the best lung. The best and most current method of estimating split lung function is to perform quantitative V/Q scan. Perfusion scans correlate better with pulmonary function. One can calculate the FEV1 volume of left over lung by knowing percentage of perfusion to left and right lung. For example:
Preoperative FEV1
Right Lung Perfusion
Left Lung Perfusion
1.5 liters
30%
70%
The tumor is in the right lung. Following resection of the right lung, we can estimate 1.5 x .7 = 1.05 liters of the left lung to remain. The minimum acceptable predicted postoperative FEV1 is 800 ml. If the predicted postoperative FEV1 volume is less than 800 milliliters the patient is not a candidate for pneumonectomy.
Step 3: If the patient has predicted post-operative FEV1 value is less than 800 ml, and if the surgeon still feels that he has a resectable lesion with a good prognosis, the next evaluation would be to occlude the pulmonary artery and measure the pulmonary artery pressure at rest and with exercise. If the pulmonary artery pressure is elevated at rest or with exercise, the patient is not a candidate for pneumonectomy. The patient obviously has no capillary bed reserve and is not able to tolerate the loss of vascular bed. He will develop cor pulmonale and the expected 5 year survival will be less than 50%. This can also be done on the operating table by clamping the pulmonary artery and measuring PA pressures.
I rarely have to go to Step 3 in my clinical practice. We need to address a few of the common questions.
PFT: Normal study. FEV1 at 2.61 L which is 100% of predicted, which is normal. FVC of 3.25 L which is 103% of predicted which is normal. FEV1/FVC ratio of 80% which is normal. Residual volume at 1.24 L which is 73% of predicted which is normal. Total lung capacity of 4.53 L which is 93% of predicted which is normal. RV/TLC ratio of 27% which is normal. Diffusion capacity at 20.8 mL/min per mmHg which is 82% of predicted, which is normal. Patient had a methacholine challenge test which was done to a final concentration of 60 mg without a drop in the FEV1 or the FVC.